Polymer-coated magnetic particle and magnetic material for absorbing electromagnetic waves

ABSTRACT

A method for producing a magnetic particle forming a magnetic material for absorbing electromagnetic waves comprises the steps of mixing an organometallic complex or a metal salt with a chain polymer and dissolving the mixture in a solvent (step S 1 ); raising the temperature of the mixture to reaction temperature (step S 2 ), carrying out a reaction at the reaction temperature (step S 3 ); and forming the magnetic particle having a structure that the periphery of each fine particle formed from the organometallic complex or the metal salt is surrounded by the chain polymer and recovering the formed magnetic particle after the reaction (step S 4 ). The magnetic particle has a nanogranular structure to become a magnetic material for absorbing electromagnetic waves. Such a magnetic particle is produced by a wet reaction. Thus, a larger amount of magnetic particle can be produced by one reaction.

The subject matter of application Ser. No. 10/245,046 is incorporatedherein by reference. The present application is a divisional applicationof U.S. Ser. No. 10/245,046, filed Sep. 17, 2002 now U.S. Pat. No.6,992,155, which claims priority to Japanese Patent Application No. JP2001-283324 filed Sept. 18, 2001 which is incorporated herein byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method for producing magneticparticles and more particularly to a method for producing magneticparticles having structures that the peripheries of fine particles aresurrounded by insulating materials.

2. Description of the Related Art

In recent years, compact and light-weight communication devices havebeen more increasingly developed as represented by portable telephones.Accordingly, parts mounted on the communication devices have beenrequested to be more compact and lighter. With the development of suchsmall-sized communication devices, operating frequencies tend to rise.

For coping with the situation that the operating frequencies of thecommunication devices are liable to high, has been carried out anattempt the electric resistance of magnetic materials used forindividual components of the communication devices such as transformers,inductors or magnetic heads is raised to reduce eddy current flowingthrough the magnetic materials themselves. As such magnetic materials,an amorphous alloy film in which metal and ceramics are sputtered at thesame time to disperse the ceramics has been proposed in Japanese PatentLaid-Open Publication No. sho 60-152651 and Japanese Patent Laid-OpenPublication No. hei 4-142710.

In a recent communication environment where the communication devicesare used mutually at close positions, the high frequencies of thecommunication devices may possibly cause a communication quality to bedeteriorated. Therefore, the components of the communication devicesemploy magnetic materials having higher magnetic permeability in a highfrequency area to absorb unnecessary radio waves generated from thecommunication devices so that the deterioration of communication qualityis prevented.

In order to realize the high magnetic permeability by such a radio waveabsorber used in the communication device, the magnetic material formingthe radio wave absorber needs to have a high saturation magnetization aswell as a high electric resistance and the anisotropic magnetic fieldand the magnetostriction of a magnetic member need to be low. In recentyears, people have paid their attention to a nanogranular structure as astructure of a magnetic material to achieve these properties at the sametime. The magnetic material has a structure that the surface of eachmagnetic particle constituting the magnetic material is surrounded by athin insulating film and these magnetic materials are connected togetherin a network form. With such a structure, grain boundary layers having ahigh resistance are formed between the magnetic particles to generate ahigh electric resistance and a high magnetic permeability is realized ina high frequency area while the magnetic particles, which are notisolated particles such as those of superparamagnetism, are brought toclose to one another.

In recent years, as for the magnetic thin film having the nanogranularstructure, Japanese Patent Laid-Open Publication No. hei 10-241938discloses that a nanogranular thin film composed of a cobalt (Co) grouphas a magnetic permeability not higher than several hundred MHz.Further, according to a report concerning a magnetic permeability (J.Appl. Phys., Vol. 87, No. 2, 15 (2000), P187), a Co alloy thin film hasa similar magnetic permeability.

However, the magnetic materials having the nanogranular structures whichhave been heretofore reported have been inconveniently limited to thinfilms using a sputtering method.

Reported values on the magnetic thin films having the nanogranularstructures have been directed only for study of the materiality of oneparticle but for bulk materials. Further, an application study utilizingthe magnetic thin film has been rarely performed. It has been especiallydifficult for the thin film to make a property of a radio wave absorberfor electromagnetic waves in neighboring places compatible with aproperty as a radio wave absorber for electromagnetic waves in remoteplaces.

Further, when the magnetic material having the nanogranular structure ismanufactured, it is estimated that a thick film not thicker than about100 μm can be manufactured by repeatedly carrying out sputteringoperations using a method for manufacturing the magnetic thin filmhaving the nanogranular structure. However, it takes high cost and longtime to produce the magnetic material, and accordingly, this method formanufacturing the magnetic material is not realistic from an industrialpoint of view.

SUMMARY OF THE INVENTION

The present invention was devised by considering the above describedproblems and it is an object of the present invention to provide amethod for producing a magnetic particle in which a magnetic particleforming a magnetic material capable of absorbing electromagnetic waves,particularly high frequency electromagnetic waves can be efficientlyproduced.

According to the present invention, there is provided a method forproducing a magnetic particle which has a surface surrounded by aninsulating material and forms a magnetic material for absorbingelectromagnetic waves. The method for producing a magnetic particlecomprises the steps of: mixing an organometallic complex or a metal saltwith a chain polymer and dissolving the mixture in a solvent; raisingthe temperature of the mixture to reaction temperature; forming themagnetic particle having a structure that the periphery of each fineparticle formed from the organometallic complex or the metal salt issurrounded by the chain polymer at the reaction temperature; andrecovering the magnetic particle.

According to the above described method, the organometallic complex orthe metal salt are mixed with the chain polymer and the mixture isdissolved in a solvent and the temperature of the mixture is raised toreaction temperalure at which a reaction is carried out. Thus, can beproduced the magnetic particle having a structure that the periphery ofeach fine particle produced from the metal of the organometallic complexor the metal salt is surrounded by the chain polymer. As describedabove, the magnetic particle forming the magnetic material for absorbingelectromagnetic waves is produced by a wet reaction, so that moremagnetic particles can be produced by one reaction.

Further, according to the present invention, there is provided amagnetic particle forming a magnetic material for absorbingelectromagnetic waves. The magnetic particle has a structure that theperiphery of each fine particle whose particle diameter is locatedwithin a range of 1 nm to 50 nm is surrounded by the chain polymer.

In case the magnetic particle having such a structure is used as themagnetic material, when respective magnetic particles are connectedtogether in a network form, there is formed a nanogranular structurethat grain boundary layers of high resistance due to the chain polymersexist between the magnetic particles. Thus, the magnetic material havinga property for absorbing electromagnetic waves can be provided.

Further, according to the present invention, there is provided amagnetic material for absorbing electromagnetic waves. The magneticmaterial comprises powder composed of a magnetic particle having fineparticles whose particle diameter ranges from 1 nm to 50 nm and whoseperipheries are surrounded by chain polymers and occupying 30% to 90% interms of volume filling rate, and a polymer material occupying the rest.

The magnetic material having the above described structure can be formedto an arbitrary shape, for instance, a sheet form, and can be applied tomaterials of various kinds of parts for absorbing the electromagneticwaves.

Still further, according to the present invention, there is provided amagnetic material for absorbing electromagnetic waves formed by pressingpowder of a magnetic particle in which the periphery of each fineparticle whose particle diameter ranges from 1 nm to 50 nm is surroundedby a chain polymer.

As described above, the powder of the magnetic particle is compressed sothat a nanogranular structure that grain boundary layers of highresistance by the chain polymers are located between the magneticparticles is formed to achieve a property for absorbing electromagneticwaves.

BRIEF DESCRIPTION OF THE DRAWINGS

The object and other objects and advantages of the present inventionwill appear more clearly from the following specification in conjunctionwith the accompanying drawings in which:

FIG. 1 is a flow chart of a method for producing a magnetic particle.

FIG. 2A-2D is a schematic diagram for explaining steps of producingmagnetic particles.

FIG. 3 is a schematic view of a reactor for producing magneticparticles.

FIG. 4 shows the measured results of the saturation magnetization andaverage particle diameter of magnetic powder obtained under reactionconditions of Examples 1 to 8.

FIG. 5 shows a relation between the reaction temperature and thesaturation magnetization obtained in the Examples 1 to 4.

FIG. 6 shows a relation between the reaction temperature and the averageparticle diameter obtained in the Examples 1 to 4.

FIG. 7 shows a relation between the reaction temperature and thesaturation magnetization obtained in the Examples 5 to 8.

FIG. 8 shows a relation between the reaction temperature and the averageparticle diameter obtained in the Examples 5 to 8.

FIG. 9 shows the measured results of the levels of electromagnetic wavesemitted from a communication device using a magnetic sheet.

FIG. 10 shows the measured results of theshield levels ofelectromagnetic waves by an IC package using a magnetic material.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Now, embodiments of the present invention will be described below byreferring to the accompanying drawings.

FIG. 1 is a flow chart of a method for producing magnetic particles. Themagnetic particles which form a magnetic material for absorbingelectromagnetic waves are produced by carrying out the steps of mixingan organometallic complex or a metal salt with a chain polymer anddissolving the mixture in a solvent (step S1); raising the temperatureof the mixture to reaction temperature while the solution is agitated(step S2), carrying out a reaction while the solution is agitated for aprescribed time at the reaction temperature (step S3); and forming themagnetic particles having a structure that the periphery of each fineparticle formed from the organometallic complex or the metal salt issurrounded by the chain polymer and recovering the formed magneticparticles after the reaction (step S4).

In the method for producing the magnetic particles, as theorganometallic complex, there are used aqueous solutions of metalcarbonyl such as iron pentacarbonyl (Fe(CO)₅), dicobalt octacarbonyl(Co₂(CO)₈) and nickel tetracarbonyl (Ni(CO)₄) or metal salts such asiron (II) chloride (FeCl₂), cobalt (II) chloride (CoCl₂), cobalt (III)chloride (COCl₃), nickel (II) chloride (NiCl₂), etc. and hydrates ofthese metal salts.

As the chain polymers, there are used poly(N-vinyl-2-pyrrolidone) havinga carbonyl group (called it “PVP”, hereinafter) or polyacrylic acid((CH₂CH(COOH))_(n)), etc. The structure of PVP is shown in abelow-described chemical formula (1).

The chain polymers to be used are suitably selected depending on desiredparticle size and reaction conditions. When PVP is used for the chainpolymer, there are employed those of molecular weight of 10000, 29000,40000 and 130000.

Here, the ratio the amount of metal of the organometallic complex or themetal salt included in a solution to the amount of chain polymersimilarly included in the solution is controlled within a range of 1:1to 1:20.

As the solvent for dissolving the organometallic complex or the metalsalt and the chain polymer, there can be used high purity alcohols orethers. As the alcohols, there may be preferably used methanol (CH₃OH),ethanol (C₂H₅OH), and propanol (C₃H₈OH). As the ether, there may bepreferably used diethyl ether ((C₂H₅)₂O). Besides, ethylene glycol((CH₂OH)₂ having boiling point of 197.85° C.), dimethyl sulfoxide((CH₃)₂S⁺O⁻) having boiling point of 189.0° C. and called it “DMSO”,hereinafter.) may be preferably employed. The structure of DMSO is shownin a below-described chemical formula (2).

Further, as the solvent, there may be preferably used hydrocarbonscapable of existing in a liquid state depending on reaction conditions,such as toluene ((C₆H₅CH₃) having boiling point of 110.6° C.), kerosine(having boiling point of 150° C. to 280° C.), etc.

The property of the magnetic material to be manufactured is slightlychanged depending on the amount of the solvent. In this embodiment, asuitable amount of the solvent indicates an amount in which theorganometallic complex or the metal salt and the chain polymer assolutes can be completely dissolved.

The organometallic complex or the metal salt is made to react with thechain polymer under prescribed conditions to produce the magneticparticles. FIG. 2 is a schematic view for explaining the steps ofproducing the magnetic particles. In this case, FIG. 2 shows an examplein which the magnetic particles are formed from the organometalliccomplex or the metal salt of an Fe group and the PVP. FIG. 2A shows astate before a reaction. FIG. 2B shows a state in which Fe atoms or Feions and the PVP are coordinated. FIG. 2C shows a state in which Feparticles grow. FIG. 2D shows a state in which the magnetic particlesare produced.

Unpaired electrons are present in oxygen (O) atoms of pyrrolidone groupsincluded in PVP 1 dissolved in the solvent and serve as Lewis acid. InFIG. 2, parts serving as the Lewis acid in the PVP 1 are designated byL.

Firstly, as shown in FIG. 2B, the Fe atoms or the Fe ions 2 likewisedissolved in the solvent through the unpaired electrons of the O atomsare coordinate-bonded to the PVP 1.

As a reaction proceeds, the Fe atoms or the Fe ions 2 coordinated withthe PVP 1 aggregate together to grow in the solvent. Then, as shown inFIG. 2C, the Fe particles 3 are formed. As the reaction furtherproceeds, the Fe particles 3 aggregate together, or the Fe particles 3and the Fe atoms or the Fe ions 2 aggregate together so that the Feparticles 3 further grow.

With the growth of the Fe particles 3, the form of the PVP 1 coordinatedwith the Fe particles 3 is also gradually changed to surround the Feparticles 3. Then, finally, is formed a magnetic particle 4 having astructure that the peripheries of the Fe particles 3 are surrounded bythe PVP 1 as shown in FIG. 2D.

The particle size of the magnetic particle 4 formed at this time isdifferent depending on the kind of a solvent to be used, reactiontemperature, reaction time and the molecular weight of the PVP 1. Theparticle size is especially greatly dependent on the kind andconcentration of the solvent. For example, when an alcohol is used as asolvent, the Fe atoms or Fe ions 2 or the Fe particles 3 are liable tobe surrounded by the PVP 1 during an initial step of the reaction.Accordingly, the growth of the Fe particles 3 is prevented. Finally, anultra-fine magnetic particle 4 of about 0.1 nm is apt to be produced. Asthe concentration of the alcohol becomes high, this tendency obviouslyappears. On the contrary, when the concentration of the solvent islowered, the magnetic particle 4 of several ten nm is finally produced.As apparent from the above-described fact, the kind and concentration ofthe solvent are properly selected, so that the particle size of themagnetic particle 4 can be conveniently controlled.

Further, the particle size of the magnetic particle 4 can be likewisecontrolled depending on whether the molecular weight of the PVP 1 islarge or small as well as the control of the particle size by thesolvent. In this case, when the molecular weight of the PVP 1 is large,much time is needed until the PVP 1 surrounds the peripheries of the Feparticles 3. During that time, since the growth of the Fe particles 3 isadvanced, the final particle size of the magnetic particle 4 isincreased.

In FIG. 2, although the example in which the Fe particles 3 whoseperipheries are surrounded by the PVP 1 are formed is described, it isto be understood that the same producing steps are likewise carried outwhen Co particles or nickel (Ni) particles are formed. Further, when theCo particles or the Ni particles are formed, the control of the particlesize depending on the kind and concentration of a solvent to be used atthat time or the control of the particle size by the molecular weight ofthe PVP 1 shows the same effects as those when the Fe particles 3 areformed.

Further, when electric field of about 10 V/cm is applied to a reactorduring the producing steps of the magnetic particle composed of theseFe, Co or Ni fine particles and the PVP, the density of the fineparticles dispersed in the solution is changed depending on the appliedtime of the electric field. When the applied time is increased, there isgenerated in the solution an area in which the density of the fineparticles is high. The use of this method makes it possible to increasethe frequency of collision between the fine particles, or between themetal atoms or metal ions and the fine particles. Consequently, theproducing speed of the magnetic particle can be accelerated. At thistime, the magnitude of the applied electric field and the applied timeare adequately selected so that the particle size of the magneticparticle or the reaction time can be controlled.

FIG. 3 is a schematic view of a reactor for producing the magneticparticle. In this embodiment, a three-neck flask 5 is used as a reactionvessel for producing the magnetic particle. An agitating vane 6 isprovided at one position of three ports in the flask 5. A nozzle 7 isprovided in one of other ports. The agitating vane 6 is rotated by amechanical agitator 8 to agitate solution with which the flask 5 isfilled. The nozzle 7 is connected to an argon (Ar) cylinder 9 disposedoutside the flask 5 to supply Ar gas to the flask 5. Further, aremaining port of the flask 5 serves as an exhaust port 10 of Ar gasentering the flask 5. In a path extending to the exhaust port 10, thereis provided a cooler 11 for allowing cooling water to flow in order toprevent vapor generated by heating the flask 5 from being emittedoutside the flask 5.

The flask 5 is disposed in a vessel 12 in which water is contained.Then, water in the vessel 12 is heated by a heater 14 powered by a powersupply 13 so that the flask 5 is substantially uniformly heated.

The magnetic particle is produced by using the reactor having theabove-described structure. Initially, the flask 5 is filled with theorganometallic complex or the metal salt, the chain polymer and thesolvent at room temperature. The Ar gas of the Ar cylinder 9 iscontinuously supplied to the flask 5 from the nozzle 7. Gas in the flask5 is replaced by the Ar gas to obtain an Ar gas atmosphere in the flask5. The gas contained in the flask 5 and the Ar gas supplied to the flask5 are exhausted from the exhaust port 10. Then, the water in the vessel12 is heated by the heater 14 to heat the liquid in the flask 5 to thetemperature of 30° C. to 50° C., preferably substantially to 40° C. sothat the organometallic complex or the metal salt and the chain polymerare completely dissolved in the solvent. After they are dissolved in thesolvent, the temperature of the obtained solution is raised by theheater 14 to a prescribed reaction temperature located between 100° C.and the boiling point of the solvent to be used. Subsequently, theagitating vane 6 is rotated by the mechanical agitator 8 to agitate thesolution in the flask 5 and perform a reaction.

After the reaction is completed, while the flask 5 is sealed, the entirebody of the flask 5 is moved to a glove box having an Ar gas atmosphereto recover a produced magnetic particle. In this case, a magnet isfirstly allowed to come near the outer wall of the flask 5 in the glovebox. Thus, since the magnetic particles dispersed in the flask 5 areattracted to the magnet, the liquid in the flask 5 is removed by, forinstance, inclining the flask 5 under this state. According to thisembodiment, a method for recovering the magnetic particles by using themagnet is called a magnetic field mineral processing. After the magneticfield mineral processing, the remaining solvent is finally completelyremoved to recover the produced magnetic particles. In order toaccelerate a drying speed, the solvent may be removed by reducingpressure in the flask 5.

Then, the average particle diameter of the obtained magnetic particlesis obtained from an X-ray diffraction measurement. Further, the particlediameter is measured under an observation by the use of a transmissionelectron microscope (TEM).

As a result of the observation by the TEM, the magnetic particlesobtained after the removal of the solvent in the flask 5 have astructure of secondary particles mutually aggregating that includesagglomerate forms such as a granular form having the diameter of 5 μm to200 μm, a flake form having the diameter of 5 μm to 200 μm and thethickness of 0.5 μm to 5 μm and a disc form having the diameter of 5 μmto 150 μm and the thickness of 0.1 μm to 5 μm. The agglomerates of themagnetic particles having the above-described forms are pulverized toobtain magnetic powder. The average particle diameter of the magneticparticles is measured under the X-ray diffraction by using the magneticpowder obtained after pulverizing. The average particle diameter isobtained in accordance with a Scherrer formula by employing the resultsof the X-ray diffraction measurement. Further, the magnetic powder isused to measure the saturation magnetization by a saturationmagnetization measurement device.

In the above description, although the Ar gas is continuously suppliedto the flask 5 to exhaust the Ar gas from the exhaust port 10, themagnetic particles may be produced in accordance with a batch type byimproving the air-tightness of a reaction vessel to be used.

Further, according to the present invention, although the magneticparticles are formed in the Ar gas atmosphere for preventing themagnetic particles to be formed from being oxidated, it is to beunderstood that gas to be used is not limited to the Ar gas, and othergas having an oxidation inhibiting effect such as other inert gas may beemployed.

EXAMPLES

Now, Examples of the present embodiment will be described below.

Example 1

Initially, Fe(CO)₅ as an organometallic complex, a PVP as a chainpolymer and DMSO as a solvent are supplied to a flask at roomtemperature. Here, Fe and PVP are dissolved in the solution in the massratio Fe:PVP=1:10. The amount of DMSO as the solvent is determined to bean amount in which Fe(CO)₅ and the PVP as solutes can be completelydissolved.

Then, Ar gas is allowed to flow in the flask to replace gas in the flaskby the Ar gas and obtain an Ar gas atmosphere. Then, a heater is heatedto obtain the temperature of liquid at 40° C. and completely dissolveFe(CO)₅ and the PVP in DMSO. After they are dissolved in DMSO, theliquid is continuously heated to reaction temperature of 130° C. andagitated to carry out a reaction.

After the reaction is completed, all the flask is directly moved to aglove box under an Ar gas atmosphere to leave produced magneticparticles in accordance with a magnetic field mineral processing in theflask and remove the rest. After that, the magnetic particles left inthe flask are completely dried and the dried magnetic particles arerecovered. The magnetic particles recovered in agglomerates arepulverized to obtain magnetic powder.

FIG. 4 shows the measured results of the saturation magnetization andthe average particle diameter of the magnetic powder obtained under thereaction conditions in the Example 1. The saturation magnetization ofthe obtained magnetic powder is 170 emu/g and the average particlediameter of the magnetic powder is 22.0 nm. Further, the particlediameter of an Fe particle at the center of the magnetic particles islocated within a range of 1 nm to 50 nm. The thickness of the PVP whichsurrounds the Fe particles can be set to 1 nm or smaller in accordancewith a calculation. Thus, the magnetic particle in which the Feparticles are surrounded by an extremely thin PVP is produced.

Example 2

Fe(CO)₅, PVP and DMSO described in the above Example 1 are used and areaction is carried out at the reaction temperature of 150° C. Theamount of materials in the solution and other procedures are the same asthose of the Example 1.

The measured results of the saturation magnetization and the averageparticle diameter of the magnetic powder obtained under the reactionconditions in the Example 2 are shown in FIG. 4. The saturationmagnetization of the obtained magnetic powder is 174 emu/g and theaverage particle diameter of the magnetic powder is 19.0 nm. Further,the particle diameter of an Fe particle at the center of the magneticparticles is located within a range of 1 nm to 50 nm.

Example 3

Fe(CO)₅, PVP and DMSO described in the above Example 1 are used and areaction is carried out at the reaction temperature of 170° C. Theamount of materials in the solution and other procedures are the same asthose of the Example 1.

The measured results of the saturation magnetization and the averageparticle diameter of the magnetic powder obtained under the reactionconditions in the Example 3 are shown in FIG. 4. The saturationmagnetization of the obtained magnetic powder is 183 emu/g and theaverage particle diameter of the magnetic powder is 16.4 nm. Further,the particle diameter of an Fe particle at the center of the magneticparticles is located within a range of 1 nm to 50 nm.

Example 4

Fe(CO)₅, PVP and DMSO described in the above Example 1 are used and areaction is carried out at the reaction temperature of 189° C. Theamount of materials in the solution and other procedures are the same asthose of the Example 1.

The measured results of the saturation magnetization and the averageparticle diameter of the magnetic powder obtained under the reactionconditions in the Example 4 are shown in FIG. 4. The saturationmagnetization of the obtained magnetic powder is 194 emu/g and theaverage particle diameter of the magnetic powder is 11.5 nm. Further,the particle diameter of an Fe particle at the center of the magneticparticles is located within a range of 1 nm to 50 nm.

Example 5

Initially, CO₂(CO)₈ as an organometallic complex, a PVP as a chainpolymer and ethylene glycol as a solvent are supplied to a flask at roomtemperature. Here, materials are dissolved in the solution in the massratio Co:PVP=1:10. The amount of ethylene glycol as the solvent isdetermined to be an amount in which CO₂(CO)₈ and the PVP as solutes canbe completely dissolved.

Then, Ar gas is allowed to flow in the flask to replace gas in the flaskby the Ar gas and obtain an Ar gas atmosphere. Then, a heater is heatedto obtain the temperature of liquid at 40° C. and completely dissolveCO₂(CO)₈ and the PVP in ethylene glycol. After they are dissolved inethylene glycol, the liquid is continuously heated to reactiontemperature of 130° C. and agitated to carry out a reaction.

After the reaction is completed, all the flask is directly moved to aglove box under an Ar gas atmosphere to leave produced magneticparticles in accordance with a magnetic field mineral processing in theflask and remove the rest. After that, the magnetic particles left inthe flask are completely dried and the dried magnetic particles arerecovered. The magnetic particles recovered in agglomerates arepulverized to obtain magnetic powder.

FIG. 4 shows the measured results of the saturation magnetization andthe average particle diameter of the magnetic powder obtained under thereaction conditions in the Example 5. The saturation magnetization ofthe obtained magnetic powder is 108 emu/g and the average particlediameter of the magnetic powder is 19.0 nm. Further, the particlediameter of a Co particle at the center of the magnetic particles islocated within a range of 1 nm to 50 nm.

Example 6

CO₂(CO)₈, PVP and ethylene glycol described in the above Example 5 areused and a reaction is carried out at the reaction temperature of 150°C. The amount of materials in the solution and other procedures are thesame as those of the Example 5.

The measured results of the saturation magnetization and the averageparticle diameter of the magnetic powder obtained under the reactionconditions in the Example 6 are shown in FIG. 4. The saturationmagnetization of the obtained magnetic powder is 111 emu/g and theaverage particle diameter of the magnetic powder is 12.0 nm. Further,the particle diameter of a Co particle at the center of the magneticparticles is located within a range of 1 nm to 50 nm.

Example 7

CO₂(CO)₈, PVP and ethylene glycol described in the above Example 5 areused and a reaction is carried out at the reaction temperature of 170°C. The amount of materials in the solution and other procedures are thesame as those of the Example 5.

The measured results of the saturation magnetization and the averageparticle diameter of the magnetic powder obtained under the reactionconditions in the Example 7 are shown in FIG. 4. The saturationmagnetization of the obtained magnetic powder is 122 emu/g and theaverage particle diameter of the magnetic powder is 10.0 nm. Further,the particle diameter of a Co particle at the center of the magneticparticles is located within a range of 1 nm to 50 nm.

Example 8

CO₂(CO)₈, PVP and ethylene glycol described in the above Example 5 areused and a reaction is carried out at the reaction temperature of 195°C. The amount of materials in the solution and other procedures are thesame as those of the Example 5.

The measured results of the saturation magnetization and the averageparticle diameter of the magnetic powder obtained under the reactionconditions in the Example 8 are shown in FIG. 4. The saturationmagnetization of the obtained magnetic powder is 113 emu/g and theaverage particle diameter of the magnetic powder is 13.0 nm. Further,the particle diameter of a Co particle at the center of the magneticparticles is located within a range of 1 nm to 50 nm.

The relations between the reaction temperatures and the saturationmagnetizations or the average particle diameters obtained in theabove-described Examples are shown in FIGS. 5 to 8. Here, FIG. 5 showsthe relation between the reaction temperatures and the saturationmagnetizations obtained in the Examples 1 to 4. FIG. 6 shows therelation between the reaction temperatures and the average particlediameters obtained in the Examples 1 to 4. FIG. 7 shows the relationbetween the reaction temperatures and the saturation magnetizationsobtained in the Examples 5 to 8. FIG. 8 shows the relation between thereaction temperatures and the average particle diameters obtained in theExamples 5 to 8.

In manufacturing the magnetic particle having the structure that theperipheries of the Fe particles are surrounded by the PVP, as thereaction temperature becomes high, the saturation magnetizationincreases as shown in FIG. 5. On the other hand, as the reactiontemperature becomes high, the average particle diameter decreases asshown in FIG. 6.

Further, upon manufacturing the magnetic particle having the structurethat the peripheries of the Co particles are surrounded by the PVP, asthe reaction temperature becomes high, it is recognized that thesaturation magnetization is liable to be increased as shown in FIG. 7,however, the degree of increase in the saturation magnetization is nothigher than that in the case of the Fe particles. On the other hand, asthe reaction temperature becomes high, it is recognized that the averageparticle diameter is apt to be decreased as shown in FIG. 8.

As described above, in the method for producing a magnetic particleaccording to this embodiment, the magnetic particle having a structurethat the surface is surrounded by the chain polymer such as the PVP canbe efficiently produced by a wet method. In case the magnetic particlehaving such a structure is employed for a magnetic material, when themagnetic particles are connected together in a network form, there isformed a nanogranular structure that grain boundary layers of highresistance due to the chain polymer exist between the magneticparticles.

In the above description, although the magnetic particle composed of Fe,Co or Ni as a single metal component is manufactured, the organometalliccomplex or the metal salt of an Fe group may be mixed with theorganometallic complex or the metal salt of a Co group and the mixturemay be used as a starting raw material to form a magnetic particlecomposed of Fe particles and Co particles. Similarly, the organometalliccomplex or the metal salt of the Fe group may be mixed with theorganometallic complex or the metal salt of an Ni group and the mixturemay be used as a starting raw material to manufacture a magneticparticle composed of Fe particles and Ni particles. At that time, theratio of components Fe to Co or Fe to Ni can be arbitrarily selected inaccordance with the characteristics or the use of a desired magneticmaterial.

Since the magnetic material composed of the magnetic particles obtainedby the above-described method has a nanogranular structure and exhibitsa property for absorbing electric waves, it can be applied to variouskinds of parts of a communication devices. Especially, since themagnetic material can absorb electromagnetic waves in a high frequencyarea, as its application form, magnetic powder as a filler may bepreferably mixed with a resin with good fluidity to obtain paste or asemiconductor mold. Further, as a high frequency package, the magneticmaterial can be likewise used for the purpose of preventing theinterference of a signal in a semiconductor device.

Further, the magnetic material has an extremely high absorbingperformance of electromagnetic waves in a low frequency area, so thatthe magnetic material can be preferably applied to uses such as anelectromagnetic shield, an inductor, a transformer, etc.

Additionally, two or more kinds of different magnetic powder are mixedtogether so that a certain property can be improved or a plurality ofkinds of characteristics can be achieved.

As one of application examples, a magnetic sheet formed by mixing themagnetic powder with a polymer material will be described below.

The magnetic sheet produced from the magnetic powder and the polymermaterial is manufactured by mixing the magnetic powder with the polymermaterial in such a manner as to include the magnetic powder having avolume filling rate of 30% to 90% (the rest is composed of the polymermaterial). As the polymer materials forming the magnetic sheet, theremay be used biodegradable polymer materials such as polylactate,poly-b-hydroxybutyrate, polybutylene succinate, polyethylene succinate,polycaprolactone, etc. Thus, there can be manufactured the magneticsheet low in its environmental load in a waste treatment, or the like.

FIG. 9 shows the measured results of levels of electromagnetic wavesemitted from a communication device using the magnetic sheet. FIG. 9shows the measured values of the levels of electromagnetic waves forrespective frequencies emitted from the device when the magnetic sheetis mounted on a casing of the communication device. For comparison, thelevels of electromagnetic waves obtained when the magnetic sheet is notmounted on the device are also measured. FIG. 9 likewise shows thedifferences between the levels of electromagnetic waves emitted when themagnetic sheet is not mounted on the communication device and the levelsof electromagnetic waves emitted when the magnetic sheet is mounted onthe communication device.

In FIG. 9, an axis of abscissa shows frequency and an axis of ordinateshows the level of electromagnetic wave emitted from the communicationdevice. The magnetic sheet is mounted on the communication device sothat the level of electromagnetic waves emitted therefrom is decreased.The emission of unnecessary electromagnetic waves from the communicationdevice can be reduced by the amount of decrease.

A magnetic sheet made of the magnetic powder and the polymer materialand having the thickness of 10 μm to 2 mm also has an effect forreducing the emission of the unnecessary electromagnetic waves.

Further, the magnetic powder and the polymer material are mixed togetherand the mixture may be used for a variety of radio wave absorber partssuch as a radio wave absorber casing, a radio wave absorbing screen, aradio wave absorbing wall, a radio wave absorbing substrate, a radiowave absorbing molded product, a radio wave absorbing package, etc. aswell as the magnetic sheet.

Further, since the magnetic powder has its surface surrounded by the PVPand insulated, a die can be directly filled with the magnetic powder tobe molded. At that time, since the magnetic particles are small, amoldability is not necessarily excellent. However, the magnetic powderis compressed by using a suitable lubricant such as stearic acid, andaccordingly, the magnetic powder can be used as the magnetic materialhaving the nanogranular structure.

This magnetic material includes the magnetic particles whose volumefilling rate is about 60% at maximum. The magnetic powder including themagnetic particles of large particle size is mixed with the magneticpowder of small particle size to obtain an optimum volume filling rateso that the volume filling rate can be further increased. For instance,the magnetic powder having the average particle diameter of 7 nm ismixed with the magnetic powder having the average particle diameter of 1nm, so that the volume filling rate of the magnetic particles of about80% can be achieved.

As an application example of such a magnetic material, an instance inwhich the magnetic material is used for an IC package will be describedbelow. FIG. 10 shows the measured results of the shield levels ofelectromagnetic waves in the IC package using the magnetic material. Forcomparison. the measured values of shield levels of electromagneticwaves by the IC package including no magnetic material are also shown inFIG. 10. In FIG. 10, the measured values obtained when the magneticmaterial is included in the IC package are shown by thick lines and themeasured values when the magnetic material is not included in the ICpackage are shown by thin lines.

Referring FIG. 10, an axis of abscissa shows frequency and an axis ofordinate shows the shield level of electromagnetic waves. When themagnetic material is included in the IC package, a shield effect ishigher throughout all frequency areas than that obtained when themagnetic material is not included in the IC package, and a highelectromagnetic wave shield effect is achieved. An electromagnetic waveshield performance in the high frequency areas is remarkably higher thanthat in a conventional case, which is specifically worthy of attention.

The above-described magnetic material can be used not only for the ICpackage, but also for a variety of filters such as a band-pass filter, ahigh-pass filter, a low-pass filter, etc. or a variety of parts of atransformer, a common mode choke coil, an inductor, etc.

As described above, according to the present invention, a magneticparticle is produced by carrying out steps of: mixing an organometalliccomplex or a metal salt with a chain polymer and dissolving the mixturein a solvent; raising the temperature of the solution to reactiontemperature; carrying out a reaction at the reaction temperature andforming the magnetic particle having a structure that the peripheries offine particles formed from the organometallic complex or the metal saltare surrounded by the chain polymer. Thus, the magnetic particlesforming the magnetic material capable of absorbing electromagnetic wavescan be efficiently produced under a wet reaction.

1. Magnetic particles forming a magnetic material for absorbing electromagnetic waves, wherein the magnetic particles have a diameter located within a range of 1 nm to 50 nm; and a chain polymer surrounding individual ones of the magnetic particles being formed to a thickness of 1 nm or less.
 2. The magnetic particles according to claim 1, wherein the magnetic particles are comprised of iron (Fe), cobalt (Co) or nickel (Ni).
 3. The magnetic particles according to claim 1, wherein the chain polymer is poly(N-vinyl-2-pyrrolidone).
 4. The magnetic particles according to claim 1, wherein the chain polymer is polyacrylic acid.
 5. A magnetic material for absorbing electromagnetic waves comprising: powder comprised of magnetic particles having a diameter of from 1 nm to 50 nm and the magnetic particles are each individually surrounded by a chain polymer that is formed to a thickness of 1 nm or less.
 6. The magnetic material according to claim 5, wherein the polymer material is a biodegradable material.
 7. The magnetic material according to claim 5, wherein the magnetic material is formed of a sheet type material having the thickness of 10 μm to 2 mm comprised of the magnetic particles each individually bonded with a chain polymer and surrounded by a further resin material.
 8. A magnetic material for absorbing electromagnetic waves formed by pressing a powder comprised of magnetic particles that are each individually surrounded by a chain polymer that is formed to a thickness of 1 nm or less. 